1. Field of the Invention
The present invention relates to data encoding and decoding systems particularly improved with respect to color pixel data.
2. Technical Background
In general, image data contains a great amount of information. If such an image data is to be processed as it is, it is not practical since an increased capacity of memory must be used with reduction of communication speed. Therefore, the compression of data is very important. The prior art have developed and practically used various data compression techniques.
One of the data compression techniques being recently attracted is the use of entropy encoder and decoder. One of the entropy encoding and decoding techniques may be of an arithmetic type as described in Japanese Patent Laid-Open Nos. Sho 62-185413, Sho 63-74324 and Sho 63-76525, for example.
FIG. 1 shows data encoding and data decoding systems 10, 20 which are constructed according to such a technique in the prior art.
The data encoding system 10 comprises a line buffer 12, and an entropy encoder 14. A color image data 100 stream is input to the line buffer 12 and entropy encoder 14 along scanning lines as pixel data, as shown in FIG. 2.
The line buffer 12 functions as a reference pixel generating means for generating reference pixels a.sub.n, b.sub.n, c.sub.n and d.sub.n for a pixel to be encoded X.sub.n from the input image data stream 100. More particularly, the line buffer 12 has stored color pixel data of n lines when an image is scanned. Each time color pixel data 100A of the pixel to be encoded X.sub.n is input into the line buffer 12, the latter outputs reference pixel data 110 toward the entropy encoder 14, the reference pixel data being a series of pixel data which include the immediately previous pixel d.sub.n and the surrounding pixels a.sub.n, b.sub.n and c.sub.n.
The entropy encoder 14 may be either of an arithmetic or to other encoder. The entropy encoder 14 uses the reference pixel data 110 as a state signal to convert the color pixel data 100A into encoded pixel data 200.
The data decoding system 20 comprises a line buffer 22 and an entropy decoder 24. The line buffer and entropy decoder 22, 24 are arranged such that they decode and output the input encoded pixel data 200 in a completely reverse manner as in the line buffer and entropy encoder 12, 14 of the arithmetic encoder 10.
More particularly, the line buffer 22 is adapted to store decoded pixel data of n lines as in the line buffer 12, and to output the reference pixel data a.sub.n, b.sub.n, c.sub.n and d.sub.n for color pixel data 100B to be next decoded toward the entropy decoder 24.
The entropy decoder 24 may be either of an arithmetic or other decoder and uses the reference pixel data 110 as a state signal to decode and output the input encoded pixel 200 as decoded color pixel data 100B.
Thus, the data encoding and data decoding systems 10, 20 use their own algorithms completely opposite to each other to encode the color pixel data 100A into the encoded pixel data 200 and also to decode the encoded pixel data 200 into the color pixel data 100B which is in turn output therefrom. Therefore, these systems can be used over a broad range of applications.
For example, the data encoding system 10 shown in FIG. 1A may be used to convert color pixel data stream 100 into an encoded pixel data stream. The encoded pixel data stream is then transmitted to the data decoding system 20 shown in FIG. 1B by any transmission means, in which the encoded pixel data stream will be decoded. Thus, a high-precision color image data transmission and reception system may be constructed which can be applied to a broad range of applications such as TV telephone, facsimile and other communications.
The data encoding system 10 shown in FIG. 1A may also be used to convert the color pixel data stream 100 into the encoded pixel data stream which can be stored in an IC chip or other. For example, if the data decoding system 20 shown in FIG. 1B is mounted in a vehicle navigation system, map data encoded by the use of the system shown in FIG. 1A may be written into any storage means such as IC chips which are in turn sold commercially. The map data may particularly contains a great amount of image data over a broad range of areas. When the map data is effectively compressed by the encoding system, the map data for large area may be written into an IC chip having a limited storage capacity and reproduced by the navigation system into which the decoding system shown in FIG. 1B is incorporated.
The system shown in FIG. 1A can compress not only static picture image data, but also dynamic picture image data. The system of FIG. 1A can compress commercial images to be reproduced through several seconds to several minutes, the images being then written into an IC chip or other means. An image display system comprising the system of FIG. 1B can be then used to reproduce these commercial images on a display. If a new product begins to be sold, the old chip including the commercial images of the previous product may only be replaced by a new IC chip which contains commercial image relating to the new product.
As described, the systems use the reference pixel data as the state signals for the entropy encoder and decoder 14, 24. Therefore, the rate of data compression can be increased if the number of states, that is, the number of reference pixels is increased.
However, the entropy encoder and decoder 14, 24 require an encoding and decoding parameter table corresponding in number to the states in the reference pixel data. As the number of reference pixels is increased to increase the rate of compression, the encoding and decoding parameter table is correspondingly enlarged. This raises a problem in that the entropy encoder and decoder 14, 24 are increased in size and cost.
It is now assumed that the color pixel data comprises four-bit data per pixel while the number of pixels in the reference pixel data 110 is equal to four. In such a case, the number of bits in the encoding and decoding parameter table will be four pixels.times.tour bits=16 bits, and the number of state will be 2.sup.16. Therefore, parameter table having number of state equal to 2.sup.16 =65,535 must be provided. From this fact, it is to be understood that the encoding and decoding parameter table will become extremely enlarged at each time when one reference pixel is added, leading to increase of the hardware dimensions in the entropy encoder and decoder 14, 24.